1. Industrial Field of the Invention
The present invention relates to a method of and an apparatus for eliminating pin holes in a block copy image, which includes characters, illustrations, logos or figures on a layout sheet, based on binary image data representing the block copy image.
2. Description of Related Art
A block copy is prepared as an original of characters and lineworks in prepress process for a color printing plate. The block copy is formed by arranging phototype-setting characters and drafted keylines on a layout sheet in the same dimensions and quality as the final products. Instructions for the subsequent processes are also given on the layout sheet; that is, the layout sheet acts as an instruction paper of prepress process.
Tint laying is generally performed in prepress process. Tint laying is processing in which a specified region within an image is uniformly filled with a desired color. Automatic tint laying is employed in some of modern image processing systems. In such systems, tint laying is completed, for example, by obtaining binary image data of a block copy image with an image scanner and filling a certain closed region in the block copy image with a desired color with an image processor. In such a conventional image processing system, an image area is filled based on a low-resolution image which is read with the scanning resolution of approximately 400 dots per inch.
Recent technological innovation has allowed the image processing system to process image data of a large data volume at a high speed. In prepress and printing, there has been a strong need for an improved apparatus which will lay tint on binary image data of a large data volume obtained at a resolution of approximately 2,000 dots per inch.
A binary image read by an image scanner often includes pin holes which are small black dots in a white area of the binary image or otherwise small white dots in a black area. Pin holes constitute a noise in a binary image and are to be eliminated for improving the quality of the image.
Such pin holes are manually eliminated one by one; an operator thoroughly checks an image displayed on a CRT (cathode ray tube) for pin holes and fills each pin hole with the color of the surrounding area. This manual processing requires a lot of time especially for an image including a large number of pin holes.
An image of a high resolution usually includes smaller sizes of and a larger number of pin holes than the image of a low resolution. A lot of time is thus required for eliminating all the pin holes in an image of a high resolution.
European Patent Application 0 194 689 to Petrick et al. discloses a method and apparatus for removing undesirable dots and voids from bit-map images by scanning the image with a window element. The size of the window element is determined by the user according to the smallest data item of a bit-map image being scanned. The data levels of the window element's outer ring are examined to determine if the levels are the same for each pixel in the outer ring. If they are, the data levels of the interior of the window are set to equal values, thus eliminating the dots or voids. However, this method of eliminating dots and voids is complex and often inaccurate.